Altitudinal Variation in the Photosynthetic Characteristics of Snow Gum, Eucalyptus pauciflora Sieb. ex Spreng. V. Rate of Acclimation to an Altered Growth Environment

Abstract

Established, field-grown, seedlings of Eucalyptus pauciflora were transferred from high- and low- elevation field sites to a controlled-environment greenhouse in Canberra (maximum/minimum daily temperature range 26/15°) and the pattern of photosynthetic acclimation observed. Levels of net photosynthesis, P_amb, intracellular resistance r_I, and leaf gas-phase resistance to CO₂ transfer (r₁) were monitored, as were the temperature optima for these parameters.

Acclimation proceeded most rapidly in the material grown at the warmer, low-elevation, site (955 m), and in the low-elevation population. Daily maximum/minimum temperatures at this site for the 10 days prior to transfer averaged 23/11°. With this material, levels of, and the temperature optimum for, P_amb reached control levels within 6 days of transfer from the field environment. By comparison, P_amb in the high-elevation population grown at the high-elevation (tree-line) site (1910 m) where the 10-day temperature averaged 15/7°, did not reach control levels until 14 days after transfer, and the temperature optimum for P_amb required 20 days to reach the control level. In general, the patterns of change in r_I and r₁ paralleled those in P_amb. Both the level of physiological activity in the field, and the temperature differences between the field and greenhouse environment, appeared to affect the rate of acclimation.

Immediately after transfer from the field, the temperature optima of the high-and low-elevation populations were close to the daily maximum temperature of the respective field environments. The temperature optimum of the high-elevation material grown at the low-elevation site was intermediate in value. At the conclusion of the acclimation period, the temperature optima of both high-elevation populations had converged to a value similar to that of the high-elevation control (about 22°); similarly, the temperature optimum of the low-elevation populations had reached the level of the low-elevation control (27°) These various temperature optima are interpreted on the basis that each population has a 'preferred' temperature which can be modified by different effective growth temperatures to yield different optima in different thermal environments. In the field, the effective temperature appears to be intermediate between the prevailing maximum and minimum temperatures.